Organismal Physiology Lecture No. 22: The Physiology Of Symbiosis
Tuesday November 27 , 2012
-Coral reefs are some of the most biodiverse ecosystems in the world as well as very productive at fixing
carbon. Symbiosis is the living together in more or less intimate association or close union of two
dissimilar organisms. In coral reefs, there is a symbiotic relationship present between an animal (coral)
and a photoautotroph (algae).
Reef-Building Corals & Zooxanthellae:
-The algae known as Zooxanthellae live within the tissue of the host coral, particularly in the gastroderm
of the animal. The Cnidrian reef-building corals contain Zooxanthellae, which allows for the calcium
carbonate skeleton. The pigments of the Zooxanthellae dictate the colour of the coral.
-Both the algae and the coral are quite dependent upon their symbiotic relationship. This is true as
tropical waters are quite nutrient-poor and the coral hosts get most of the energy produced as
photosynthate from the Zooxanthellae (up to 90%) in this cyclical relationship. In exchange, the coral
provides inorganic nutrients that the algae needs for photosynthesis, one of these being ammonia (NH ),
which is actually the main production of waste by the host. The products of photosynthesis are handed
over to the host coral since it secretes a molecule known as Host Transmission Factor (HTF) that
stimulates the release of compounds from algae. The presence of this factor increases the amount of
lactate, alanine and amino acids as well as gets them transferred them to the host.
Benefits & Downfalls Of The Symbiosis:
-The algae symbionts benefit the coral by providing up to 90%of host nutrition as well as act as a sink for
potentially toxic nitrogenous waste compounds (ammonia). The coral benefits the algae by having
access to inorganic nutrients from the host as a source of growth and a substratum deployed in the sun
to permit optimal photosynthesis. The cost to the host coral is that the symbionts act as a source of